Abstract: Prorocentrum is a kind of very common dinoflagellate species in China coastal waters,and P.donghaiense Lu is the most key bloom causative species in East China Sea in recent years.Better understanding and analysis about the dynamics of Prorocentrum population in Nanji Islands Sea Area may support many useful data and provide helpful guides for the ecological studies and protection of Nanji Islands National Nature Reserve.A series of water samples were collected from four stations in Nanji Islands National Nature Reserve from April 2006 to March 2007.During this investigation period,monthly samples were gathered from July 2006 to March 2007,which including and corresponding the summer,autumn and winter periods.And in the spring,from April to June,2006,in which period the Prorocentrum blooms occurred frequently in recent several years,water samples were collected nearly every week.Both surface and bottom samples were got respectively.Species composition,seasonal variations and vertical distribution of Prorocentrum population as well as several chemical parameters were investigated.And the connections between Prorocentrum dynamics and several environmental factors,such as water temperature,salinity,nitrogen and phosphorus nutrition concentration,were analyzed.Totally,four Prorocentrum species have been found,P.triestinum Schiller,P.donghaiense Lu,P.micans Ehrenberg and P.sigmoides Bohm,in which,P.triestinum and P.donghaiense were the dominant species of phytoplankton,especially in spring and induced blooms in 12 May and 25 May respectively.During Prorocentrum blooms periods,Prorocentrum cell abundances distributing in the surface water levels were much higher than that in the bottom water levels.It was the result that Prorocentrum cells had the ability to move anywhere through their flagella swinging.So they could shift their vertical distribution based on different times in a day,moving to and distributing near the upper water levels in the daylight for better sunshine and removing to the bottom water levels during the dark.Peak abundances of P.triestinum occurred under the water temperature 18.5-19.5℃ and salinity 29.58‰-30.95‰.And the peak abundances of P.donghaiense occurred under the water temperature 18.5-21.3℃ and salinity 29.03‰-30.92‰.Higher Prorocentrum population cell abundances corresponded to both lower phosphorus and nitrogen nutrition concentrations.Using correlate analysis by SPSS,it is revealed that the dynamics of Prorocentrum population has obvious positive correlation with the ratio of DIN to PO4-P,and obvious negative correlation with the phosphorus concentrations.The phosphorus concentration is the key role influencing on the variation of Prorocentrum population.The nutritional storage ability of Prorocentrum cells is much higher than that of many diatom species.Maybe it is the actual reason why Prorocentrum cells could grow better and cause blooms under lower nutrition concentration in spring in Nanji Islands National Nature Reserve.